Due to recent events world wide, there is a high level of interest in the detection of low level concentrations of explosives such as TNT (2,4,6-trinitrotoluene) and RDX (cyclotrimethylenetrinitramine) for the identification of individuals and areas exposed to these compounds. Detection of these compounds is also of interest for monitoring levels in soil and ground water near sites of munitions handling and storage. Several U.S. Department of Defense (DoD) and former DoD munitions handling sites have elevated concentrations of explosives and/or volatile organic compounds (VOCs) in soil and groundwater (U.S. Army Environmental Center, Remediation of explosives contaminated soil, http://aec.army.mil/usaec/technology/cleanup01.html 2003; Crockett et al. Field sampling and well selecting on-site analytical methods for explosives in water,” Rep. No. 600/S-99/002. U.S. Environmental Protection Agency, 1999). VOCs are also found in a wide range of consumer products including paints and cleaning supplies and are produced by chemical manufacturing processes, automotive exhaust, and the evaporation of petroleum-based products (U.S. Environmental Protection Agency, USEPA, U.S. Consumer Product Safety Commission. 1995. The Inside Story: A Guide to Indoor Air Quality. U.S. Environmental Protection Agency Document #402-K-93-007, 1995). Exposure to TNT or RDX can result in skin, eye, and respiratory tract irritation; nervous system irregularities; and convulsions. Exposure to high levels of VOCs can result in irritation of mucous membranes, headaches, nausea, damage to liver or kidneys, and is suspected of increasing the risk of certain types of cancer (USEPA, 1995).
Dogs are currently the choice for sensitive, broad-spectrum detection of explosives (Haupt et al., Applicability of Portable Explosive Detection Devices in Transit Environments,” Rep. No. 86. Transportation Research Board of the National Academies., Washington, D.C., 2004). They are capable of detection of a wide range of analytes including explosives, fuels, and even bio-threats at exceptionally low levels: they are resistant to masking interferents; and they are able to spatially locate the source. Dogs also provide additional security in the form of a deterrent. Unfortunately, dogs fatigue after working for short periods of time and their training and upkeep is expensive requiring a dedicated handler. Ion mobility spectrometry is based on the time required for various ions of a vaporized and ionized sample to reach the detector (Fetterolf and Clark, Detection Of Trace Explosive Evidence By Ion Mobility Spectrometry, Journal of Forensic Sciences 38, 28-39, 1993; Garofolo et al., Rapid Communications in Mass Spectrometry 8, 527-532, 1994; Koyuncu et al., Turkish Journal of Chemistry 29, 255-264, 2005; Pen et al., Solid phase microextraction ion mobility spectrometer interface for explosive and taggant detection, Journal of Separation Science 28, 177-183, 2005). Ion mobility spectrometry lends itself to the development of portable instruments as it is sensitive and does not require ionization under vacuum. Limitations include false alarms due to similar drift times of non-threat agents and lack of quantitative capability. Electron capture detection is similar to ion mobility spectrometry (Baffle et al., Enhanced Detection of Nitroaromatic Explosive Vapors Combining Solid-phase Extraction-Air Sampling, Supercritical Fluid Extraction and Large Volume Injection-GC, Analytical Chemistry 75, 3137-3144, 2003; Monteil-Rivera et al., Use of solid-phase microextraction/gas chromatography—electron capture detection for the determination of energetic chemicals in marine samples, Journal of Chromatography A 1066, 177-187, 2005; Zhang et al., Use of pressurized liquid extraction (PLE)/gas chromatography-electron capture detection (GC-ECD) for the determination of biodegradation intermediates on hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in soils, Journal of Chromatography B-Analytical Technologies in the Biomedical and Life Sciences 824, 277-282, 2005). These detectors measure the affinity of the sample for electrons. False positive rates are reported to be higher than those of ion mobility spectrometry (Haupt et al., 2004); individual explosives cannot be identified by electron capture detection; and compressed gases such as Helium or Argon are required. High performance liquid chromatography and mass spectrometry are well established, high sensitivity techniques for the detection of explosives, but are suitable for use in laboratory settings primarily.
There are several new technologies being developed for explosives detection: surface acoustic wave (Grate, Wave Microsensor Arrays for Vapor Sensing, Chemical Reviews 100, 2627-2647, 2000; Houser et al., Rational materials design of sorbent coatings for explosives: applications with chemical sensors, Talanta 54, 469-485, 2001; Kannan et al., Detection of Landmine Signature using SAW-based Polymer-coated Chemical Sensor, Defense Science Journal 54, 309-315, 2004), semiconducting organic polymers, and amplifying fluorescent polymers. Detection by surface acoustic wave sensors is based on a change in frequency which corresponds to a change in mass caused by adsorption of target analyte by a polymer surface. Surface acoustic wave sensors can employ an array of polymers allowing discrimination of a range of analytes as well as providing reduction of false positives. These sensors are somewhat sensitive to temperature fluctuations and tend to be humidity sensitive. Polymer surfaces are regenerable. Semi-conducting organic polymers provide highly sensitive detection of electron deficient nitroaromatics such as TNT (Rose et al., Sensitivity gain in chemosensing by lasing action in organic polymers, Nature 434, 876-879, 2005). Detection is based on quenching of polymer fluorescence upon analyte binding. Exceptional detection limits under ambient conditions have been reported, however, pulsed laser excitation is required and the polymers are subject to photo-bleaching. The technique is limited to NO2-containing compounds and is subject to interference by non-threat compounds of similar structure. Amplifying fluorescent polymers use a molecular-wire with multiple binding sites and multiple fluorophores (Cumming et al., Using Novel Fluorescent Polymers as Sensory Materials for Above-Ground Sensing of Chemical Signature Compounds Emanating from Buried Landmines, IEEE Transactions on Geoscience and Remote Sensing 39, 1119-1128, 2001). Interaction of a target molecule with a single binding site results in quenching of many fluorophores providing an amplifying effect. Laser excitation is not necessary. TNT and compounds of similar structure such as dinitrotoluene can be detected, though without discrimination. Detection is accomplished in real-time and polymers are regenerable/reusable. Other types of detection techniques employ proteins such as antibodies and enzymes for the specific recognition of target molecules.
PMOs are organic-inorganic polymers with highly ordered pore networks and large internal surface areas. They were first reported in 1999 (Inagaki et al. Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks, J. Am. Chem. Soc. 1999, 121,9611; Asef et al., Periodic mesoporous organosilicas with organic groups inside the channel walls, Nature 1999, 402, 867; Melde et al., Mesoporous Sieves with Unified Hybrid Inorganic/Organic Frameworks, Chem. Mater. 1999, 11, 3302), these organosilicas were synthesized using a surfactant template approach (Kreseg et al., Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature, 359, 710, 22 Oct. 1992, Burleigh et al., Direct synthesis of periodic mesoporous organosilicas: Functional Incorporation by Co-Condensation with Organosilanes, J. Phys. Chem. B 2001, 105, 9935) and have narrow pore size distributions with few blocked pores or obstructions commonly found in amorphous materials to impede molecular diffusion throughout their pore networks. PMOs possess structural rigidity arising from the siloxane groups and functionality due to the organic bridging group. In addition, specificity can be imparted to the PMOs via a template directed molecular imprinting process. Due to their structural stability, functionality, and specificity, the PMOs are very efficient sorbents for the removal, sequestration, and pre-concentration of pollutants and/or any targeted compound from both vapor and aqueous phase. Yet a secondary means, such as a spectroscopic or electrochemical technique, is required for the specific detection of the sorbate. The addition/embedding of molecules (ie fluorophores) would in effect add a sensing capability to PMOs through the spectrophotometric response of the embedded molecule to changes in its surrounding environment.
PMOs are hybrid materials containing organic functionality in a silica matrix through covalent silica-carbon bonds which serve as an “organic bridge” within the wall of the matrix. These materials possess relatively high surface areas with high organic loading which can be modified by incorporating different organic bridging materials or multiple organic groups (Jayasundera et al., Organosilica Copolymers for the Adsorption and Separation of Multiple Pollutants, J. Phys. Chem. B 109, 9198-9201, 2005). The structure of the material provides a high internal surface area as well as narrow pore size distribution. Template directed molecular imprinting, the use of a target-like compound, can be used to produce more homogeneous pore size and distribution as well as to further enhance binding characteristics and selectivity. PMO materials have been described for use in environmental clean-up for the adsorption of toxic chemicals from water sources (Burleigh et al., Porous Polysilsesquioxanes for the Adsorption of Phenols, Environ Sci Technol 36, 2515-18, 2002) and, with photochemical modification, for potential use in switches and sensors (Alvaro et al., Photochemical modification of the surface area and tortuosity of a trans-1,2-bis(4-pyridyl)ethylene periodic mesoporous MCM organosilica, Chem. Commun, 2012-13, 2002; Mal et al., Photocontrolled reversible release of guest molecules from coumarin-modified mesoporous silica, Nature 421, 350-3, 2003). The PMOs can be optimized for adsorption of TNT and similar compounds, however, PMO materials can be synthesized for adsorption of many materials, including, but not limited to, chemical warfare agents, pesticides, volatile organic compounds, or toxic industrial products. Optimization of the PMO materials is disclosed in Markowitz, et al., U.S. patent application Ser. No. 11/307,286, filed Jan. 31, 2006, incorporated herein in full by reference. The use of periodic mesoporous organosilicas (PMOs) as recognition elements offers advantages in stability, selectivity, and ease of modification.
The molecular structure of the porphyrin consists of a large macrocycle around which a minimum of 22 π-electrons are shared, resulting in a high degree of sensitivity to the immediate environment of the molecule. This large number of π-electrons results in a large extinction coefficient and spectral characteristics that are highly sensitive to changes in the environment of the molecule. In general, porphyrins possess a strong absorption band around 400 nm with an extinction coefficient that can exceed 500 mM-1·cm-1 as well as several less intense bands between 450 and 700 nm. Porphyrins are typically intensely fluorescent with emission bands between 600 and 750 nm. Porphyrins have been used in a wide range of optical detection applications (Malinski, The Porphyrin Handbook, K. M. Kadish, K. M. Smith and R. Guilard, (Eds.). Vol. 6, pp. 231. Academic Press, New York, 2000) and the sensitivity of porphyrin spectrophotometric characteristics to the presence of cyclic organics has been demonstrated by several groups (Mauzerall, Spectra of molecular complexes of porphyrins in aqueous solution, Biochemistry 4, 1801-1810, 1965; Rakow and Suslick, A colorimetric sensor array for odour visualization, Nature 406, 710-713, 2000; Schneider and Wang, Ligand-Porphyrin Complexes: Quantitative Evaluation of Stacking and Ionic Contributions, J. Org. Chem 59, 7464-7472, 1994; Shelnutt, Molecular complexes of copper uroporphyrin with aromatic acceptors, J. Phys. Chem 87, 605-616, 1983; Umar et al., Self-assembled monolayer of copper(II) meso-tetra(4-sulfanatophenyl) porphyrin as an optical gas sensor, Sensors and Actuators B 101, 231-235, 2004).
The changes in spectral characteristics of the porphyrin are specific for interaction with different molecules allowing for discrimination of analytes even within closely related structures such as amino acids of differing chirality. Because of these unique characteristics, porphyrins are used in a wide variety of sensor applications for the detection of analytes ranging from metal ions and volatile organic compounds to proteins. Recently arrays of different porphyrins have incorporated in fluorescence-based electronic noses. Recent work (White, et al., Reagent-less detection of a competitive inhibitor of immobilized acetylcholinesterase, BiosenBioelec 2002, 17, 361) has shown that porphyrins can be used in conjunction with enzymes to achieve a higher degree of selectivity and allow for specific detection within a class of compounds only. The reversible, competitive inhibition of an enzyme by a porphyrin has been used for the detection both in solution and vapor phase of analytes such as organophosphates (including nerve agents/stimulants) and carbon dioxide (White, et al., Enzyme-based detection of Sarin (GB) using planar waveguide absorbance spectroscopy, SensLett 2005, 3, 36 and White, et al., Competitive Inhibition of Carbonic Anhydrase by Water Soluble Porphyrins: Use of carbonic anhydrase as a CO2 Sensor, SensLett 2005, 3, 59). Porphyrins have been previously used in a detection scheme using a flat-bed scanner (Rakow and Suslick, A colorimetric sensor array for odour visualization, Nature 406, 710-713, 2000), however the porphyrins were immobilized in gel, which leads to a lower degree of selectivity than porphyrins embedded in a PMO material.
One object of the present invention is to provide for a detection scheme that utilizes both the exceptional surface area and selectivity of the PMOs as well as the spectrophotometric characteristics of a fluorophore, i.e. a porphyrin, that allows rapid, specific detection of volatile organic compounds in aqueous solution and in vapor phase. Another object is to provide a detection scheme where the spectrophotometric responses of the materials to various organics at high concentrations are discernable by visual inspection of the imprinted material as well as by standard spectrophotometric techniques. A further object is to provide a material that is capable of detection/discrimination of nitroenergetics and closely related compounds in aqueous solution. These and other objects are provided by the invention disclosed.